Note that in ECMA-262, edition 3 (IE7, IE8 etc.) it is not possible to use methods that have keyword names like .catch and .finally. The API documentation always lists a compatible alternative name that you can use if you need to support these browsers. For example .catch is replaced with .caught and .finally with .lastly.

Also, long stack trace support is only available in Chrome, Firefox and Internet Explorer 10+.

If you find a bug in bluebird or have a feature request, file an issue in the github issue tracker. Anything else, such as questions for help in using the library, should be posted in StackOverflow under tags promise and bluebird.

This is a problem every promise library needs to handle in some way. Unhandled rejections/exceptions don't really have a good agreed-on asynchronous correspondence. The problem is that it is impossible to predict the future and know if a rejected promise will eventually be handled.

There are two common pragmatic attempts at solving the problem that promise libraries do.

The more popular one is to have the user explicitly communicate that they are done and any unhandled rejections should be thrown, like so:

download().then(...).then(...).done();

For handling this problem, in my opinion, this is completely unacceptable and pointless. The user must remember to explicitly call .done and that cannot be justified when the problem is forgetting to create an error handler in the first place.

The second approach, which is what bluebird by default takes, is to call a registered handler if a rejection is unhandled by the start of a second turn. The default handler is to write the stack trace to stderr or console.error in browsers. This is close to what happens with synchronous code - your code doesn't work as expected and you open console and see a stack trace. Nice.

Of course this is not perfect, if your code for some reason needs to swoop in and attach error handler to some promise after the promise has been hanging around a while then you will see annoying messages. In that case you can use the .done() method to signal that any hanging exceptions should be thrown.

If you want to override the default handler for these possibly unhandled rejections, you can pass yours like so:

Promise.onPossiblyUnhandledRejection(function(error){

throw error;

});

If you want to also enable long stack traces, call:

Promise.longStackTraces();

right after the library is loaded.

In node.js use the environment flag BLUEBIRD_DEBUG:

BLUEBIRD_DEBUG=1 node server.js

to enable long stack traces in all instances of bluebird.

Long stack traces cannot be disabled after being enabled, and cannot be enabled after promises have already been created. Long stack traces imply a substantial performance penalty, even after using every trick to optimize them.

A practical problem with Promises/A+ is that it models Javascript try-catch too closely for its own good. Therefore by default promises inherit try-catch warts such as the inability to specify the error types that the catch block is eligible for. It is an anti-pattern in every other language to use catch-all handlers because they swallow exceptions that you might not know about.

Now, Javascript does have a perfectly fine and working way of creating error type hierarchies. It is still quite awkward to use them with the built-in try-catch however:

try{

//code

}

catch(e){

if( e instanceof WhatIWantError){

//handle

}

else{

throw e;

}

}

Without such checking, unexpected errors would be silently swallowed. However, with promises, bluebird brings the future (hopefully) here now and extends the .catch to accept potential error type eligibility.

For instance here it is expected that some evil or incompetent entity will try to crash our server from SyntaxError by providing syntactically invalid JSON:

getJSONFromSomewhere().then(function(jsonString){

return JSON.parse(jsonString);

}).then(function(object){

console.log("it was valid json: ", object);

}).catch(SyntaxError,function(e){

console.log("don't be evil");

});

Here any kind of unexpected error will automatically reported on stderr along with a stack trace because we only register a handler for the expected SyntaxError.

Ok, so, that's pretty neat. But actually not many libraries define error types and it is in fact a complete ghetto out there with ad hoc strings being attached as some arbitrary property name like .name, .type, .code, not having any property at all or even throwing strings as errors and so on. So how can we still listen for expected errors?

Bluebird defines a special error type OperationalError (you can get a reference from Promise.OperationalError). This type of error is given as rejection reason by promisified methods when
their underlying library gives an untyped, but expected error. Primitives such as strings, and error objects that are directly created like new Error("database didn't respond") are considered untyped.

Example of such library is the node core library fs. So if we promisify it, we can catch just the errors we want pretty easily and have programmer errors be redirected to unhandled rejection handler so that we notice them:

//Read more about promisification in the API Reference:

//API.md

var fs = Promise.promisifyAll(require("fs"));

fs.readFileAsync("myfile.json").then(JSON.parse).then(function(json){

console.log("Successful json")

}).catch(SyntaxError,function(e){

console.error("file contains invalid json");

}).catch(Promise.OperationalError,function(e){

console.error("unable to read file, because: ", e.message);

});

The last catch handler is only invoked when the fs module explicitly used the err argument convention of async callbacks to inform of an expected error. The OperationalError instance will contain the original error in its .cause property but it does have a direct copy of the .message and .stack too. In this code any unexpected error - be it in our code or the fs module - would not be caught by these handlers and therefore not swallowed.

Since a catch handler typed to Promise.OperationalError is expected to be used very often, it has a neat shorthand:

Finally, Bluebird also supports predicate-based filters. If you pass a
predicate function instead of an error type, the predicate will receive
the error as an argument. The return result will be used to determine whether
the error handler should be called.

Predicates should allow for very fine grained control over caught errors:
pattern matching, error typesets with set operations and many other techniques
can be implemented on top of them.

Example of using a predicate-based filter:

var Promise =require("bluebird");

var request = Promise.promisify(require("request"));

functionclientError(e){

return e.code>=400&& e.code<500;

}

request("http://www.google.com").then(function(contents){

console.log(contents);

}).catch(clientError,function(e){

//A client error like 400 Bad Request happened

});

Danger: The JavaScript language allows throwing primitive values like strings. Throwing primitives can lead to worse or no stack traces. Primitives are not exceptions. You should consider always throwing Error objects when handling exceptions.

ReferenceError: a is not defined
at evenMoreInner (<anonymous>:7:13)
From previous event:
at inner (<anonymous>:6:20)

Compare to:

Error.stackTraceLimit =25;

Promise.longStackTraces();

Promise.resolve().then(functionouter(){

return Promise.resolve().then(functioninner(){

return Promise.resolve().then(functionevenMoreInner(){

a.b.c.d()

}).catch(functioncatcher(e){

console.error(e.stack);

});

});

});

ReferenceError: a is not defined
at evenMoreInner (<anonymous>:7:13)
From previous event:
at inner (<anonymous>:6:36)
From previous event:
at outer (<anonymous>:5:32)
From previous event:
at <anonymous>:4:21
at Object.InjectedScript._evaluateOn (<anonymous>:572:39)
at Object.InjectedScript._evaluateAndWrap (<anonymous>:531:52)
at Object.InjectedScript.evaluate (<anonymous>:450:21)

The value of boolean flags is determined by presence, if you want to pass false value for a boolean flag, use the no--prefix e.g. --no-browser.

--run=String. Which tests to run (or compile when testing in browser). Default "all". Can also be a glob string (relative to ./test/mocha folder)

--cover=String. Create code coverage using the String as istanbul reporter. Coverage is created in the ./coverage folder. No coverage is created by default, default reporter is "html" (use --cover to use default reporter).

--browser - Whether to compile tests for browsers. Default false.

--port=Number - Whe port where local server is hosted when testing in browser. Default 9999

--execute-browser-tests - Whether to execute the compiled tests for browser when using --browser. Default true.

Currently the most relevant benchmark is @gorkikosev's benchmark in the article Analysis of generators and other async patterns in node. The benchmark emulates a situation where n amount of users are making a request in parallel to execute some mixed async/sync action. The benchmark has been modified to include a warm-up phase to minimize any JITing during timed sections.

Make sure you have cloned the repo somewhere and did npm install successfully.

After that you can run:

node tools/build --features="core"

The above builds the most minimal build you can get. You can add more features separated by spaces from the above list:

node tools/build --features="core filter map reduce"

The custom build file will be found from /js/browser/bluebird.js. It will have a comment that lists the disabled and enabled features.

Note that the build leaves the /js/main etc folders with same features so if you use the folder for node.js at the same time, don't forget to build
a full version afterwards (after having taken a copy of the bluebird.js somewhere):

Building a library that depends on bluebird? You should know about a few features.

If your library needs to do something obtrusive like adding or modifying methods on the Promise prototype, uses long stack traces or uses a custom unhandled rejection handler then... that's totally ok as long as you don't use require("bluebird"). Instead you should create a file
that creates an isolated copy. For example, creating a file called bluebird-extended.js that contains:

//NOTE the function call right after

module.exports =require("bluebird/js/main/promise")();

Your library can then use var Promise = require("bluebird-extended"); and do whatever it wants with it. Then if the application or other library uses their own bluebird promises they will all play well together because of Promises/A+ thenable assimilation magic.

You should also know about .nodeify() which makes it easy to provide a dual callback/promise API.

The sync build is provided to see how forced asynchronity affects benchmarks. It should not be used in real code due to the implied hazards.

The normal async build gives Promises/A+ guarantees about asynchronous resolution of promises. Some people think this affects performance or just plain love their code having a possibility
of stack overflow errors and non-deterministic behavior.

This should pressure the CPU slightly less and thus the sync build should perform better. Indeed it does, but only marginally. The biggest performance boosts are from writing efficient Javascript, not from compromising determinism.

Note that while some benchmarks are waiting for the next event tick, the CPU is actually not in use during that time. So the resulting benchmark result is not completely accurate because on node.js you only care about how much the CPU is taxed. Any time spent on CPU is time the whole process (or server) is paralyzed. And it is not graceful like it would be with threads.

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The above copyright notice and this permission notice shall be included in
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